A composite polymer membrane, which comprises a microporous matrix formed from a first polymeric material and a second polymeric material within the pores of the matrix, can be used as a selective barrier. For example, such a membrane may be used to separate components of a liquid. By use of an ion exchange material as the second polymeric material, such a membrane may be used as a selective barrier for ions in solution; for example, such a membrane may be used as an electrode separator in an electrochemical device, in which ionic conduction between electrodes of the device is required and in which a barrier is required to prevent migration of certain dissolved species (such as ions) and of electrode material in particulate form. The barrier is also required in secondary cells on recharging, when loosely attached material is deposited on the anode, often in the form of dendrites, which is required not to make contact with the cathode. A composite polymer membrane can be used to separate materials by other mechanisms, for example by ultrafiltration.
A suitable composite polymer membrane for use as an electrode separator in an electrochemical device is disclosed in our U.S. Pat. No. 5,256,503 the disclosure of which is incorporated herein by reference thereto. The membrane comprises a porous matrix of polyethylene which has been formed from a blend of polyethylene and polyethylene oxide ;by removal of the polyethylene oxide.
The pores in the matrix contain polyacrylic acid which preferably has been polymerized in situ in the pores, and which blocks, and preferably fills, the pores of the matrix. The acrylic acid is supplied to the pores of the matrix in solution with a crosslinking agent and a photoinitiator. Polymerization and crosslinking of the acrylic acid are initiated by exposure to ultraviolet radiation.
The polymerization and crosslinking reactions of the acrylic acid in the composite membrane disclosed in U.S. Pat. No. 5,256,503 compete for the acrylic acid, and by selecting the relative rates of the competing reactions, it is possible to control the degrees of crosslinking and polymerization of the acrylic acid. It has been found possible to vary the properties of the known membrane by appropriate selection of the crosslink density of the acrylic acid impregnant, for example to provide a membrane with a low resistance and relatively weak barrier performance using a low crosslink density, and to provide a membrane with relatively high resistance and good barrier performance using a high crosslink density. The degree of crosslinking can be selected by for example providing the impregnant with an appropriate quantity of crosslinking agent for reaction. However, a remaining difficulty lies in producing a membrane by the technique disclosed in U.S. Pat. No. 5,256,503 which has a good barrier performance towards certain dissolved species (such as silver or mercury ions).
Another approach to producing a composite polymer membrane is the subject of U.S. Pat. No. 3,427,206, and involves grafting a polymer of an ethylenically unsaturated carboxylic acid such as acrylic acid onto a non-porous sheet of a material such as polyethylene. This approach can be contrasted to that discussed above, in that ion migration takes place through the bulk polymer of the sheet to which acrylic acid has been grafted, rather than through regions of polyacrylic acid contained within pores in a microporous sheets.
Membranes which are formed by grafting an ion exchange material onto the polymeric material of a continuous sheet may be crosslinked, generally before the ion exchange material is supplied. It has been found that crosslinking of such membranes has effects which are similar to those in the membranes disclosed in WO-A-87/06395, affecting ionic conductivity through the membrane and barrier performance. This can be understood since crosslinks are provided in both systems effectively between molecules of ion exchange material, the material in the first system described above being bulk material, whereas that in the second system described above is grafted onto the polymeric material of a continuous sheet.
Polymer membranes of the type disclosed in WO-A-87/06395 which comprises a porous matrix of a first polymeric material, with a second polymeric material within its pores, have been found to have advantages compared with membranes which comprise a continuous sheet of a first polymeric material with a second material grafted to the material of the sheet. For example, the ionic conductivity through a filled microporous sheet has been found to be higher than that through a grafted sheet.